Spacecraft perform various maneuvers after they are launched into space and once they are on-station in a selected orbit. For example, after a spacecraft is launched into a low orbit, it may be required to raise the spacecraft to a selected higher (e.g., Geosynchronous) orbit by firing the spacecraft's main thruster. This type of maneuver is known as an orbit-raising maneuver. Also by example, after the spacecraft is on-station in the higher orbit, various forces (e.g., solar forces) may impinge on the spacecraft and cause the spacecraft to drift away from its selected orbit into another incorrect orbit. Thus, periodic (e.g., monthly) orbital maneuvers are often required to return the spacecraft to its correct orbit. These types of maneuvers are known as station-keeping maneuvers.
During each of the maneuvers, the controlling of the spacecraft's attitude to orient the spacecraft's communication hardware to a preselected planetary location and/or to correctly orient the spacecraft's thrust vector is essential. Thus, spacecraft are typically equipped with control systems which enable the attitude of the spacecraft to be controlled within pre-established limits. Such control systems often employ spacecraft thrusters for producing torques on the spacecraft, if needed, for correcting the spacecraft attitude. By example, during orbit-raising maneuvers, attitude control can be maintained by activating selected ones of the spacecraft's thrusters to create a desired torque in order to correct the spacecraft's attitude. Since the spacecraft's thrusters are normally-off during these maneuvers, and are only activated when needed, these thrusters function in an "on-modulation mode" and may be referred to as "on-modulated thrusters". Also by example, during station-keeping maneuvers, where selected ones of the spacecraft's thrusters are normally-on for performing station-keeping functions, these thrusters can be used to achieve attitude control in order to create a desired torque on the spacecraft which will correct the spacecraft's attitude.
There are several spacecraft systems used for controlling station-keeping and attitude including, by example, one system which may be understood in view of the block diagram of a spacecraft thruster control system shown in FIG. 1. Sensors 1, which may be, for example, earth sensors, sense the spacecraft dynamics, such as the attitude of the spacecraft during a maneuver, and provide the sensed information to a controller block 3 via a hardware (H/W) and software (S/W) interface 2. The controller block 3 determines whether there is error between the sensed attitude and a desired or reference attitude and, if there is such an error, calculates torques necessary to be produced by selected thrusters 6 to minimize the error. A duty cycle for the thrusters is then calculated based upon the calculated torques and an amount of torque known to be produced by the selected thrusters when the thrusters are activated. A thruster hardware control block 5 provided and is responsive to receiving a pulse from a modulator block connected to the controller for controlling an appropriate thruster corresponding to the pulse. Once the thruster is commanded to activate, valves of the thruster emit a stored fuel and a stored oxidizer which combust upon contacting one another to generate a force of a known magnitude.
In the past the control system for controlling the coordinated On and OFF conditions of one or more thrusters to effect such movements of the spacecraft in space required the manipulation of large matrixes including the multiplication thereof, even in spacecraft wherein only four thrusters are used. These types of spacecraft are usually the less-expensive spacecraft. One prior art patent which illustrates such known control systems which are usable with such spacecraft is set forth in U.S. Pat. No. 5,140,525. This system is driven by established methods in constrained optimization theory which involve among other things, weighted pseudo inversions of torque matrixes, etc., requiring the use of a highly memory intensive flight computer on board the spacecraft.
Reference may be had to other patents which involve similar satellite controls as follows:
______________________________________ 3,866,025 Cavanagh 4,537,375 Chan 4,599,697 Chan et al. 4,758,957 Hubert et al. 4,961,551 Rosen ______________________________________
Accordingly it is an object of the present invention to provide a satellite control system which does not use either pseudo-inverses nor matrix inversions nor any matrix operations.
It is a further object of the invention to provide a satellite control system and thruster arrangement aligned optimally prior to spacecraft fabrication so as to guarantee positive ON-times for all four thrusters, independent of the sign and/or magnitude of the desired torques.
It is a further object of the invention to provide a control system which effects satellite control of the aforementioned type accomplished through non-linear computer coded logic that is easy to implement.